High cpu and memory usage with lens typed holes

[farmerzhang1]

Your environment

WSL2 Ubuntu 20.04

vscode-haskell

my cabal file

cabal-version:      2.4
name:               hls-lens-hole
version:            0.1.0.0

executable hls-lens-hole
    main-is:          Main.hs
    build-depends:    base ^>=4.14.3.0, lens
    hs-source-dirs:   app
    default-language: Haskell2010

ghc version

The Glorious Glasgow Haskell Compilation System, version 8.10.7

cabal

cabal-install version 3.6.2.0
compiled using version 3.6.2.0 of the Cabal library

HLS version (vscode-haskell installs the latest version automatically)

ls ~/.vscode-server/data/User/globalStorage/haskell.haskell/
haskell-language-server-wrapper-1.6.1.0-linux
haskell-language-server-1.6.1.0-linux-8.10.7

Steps to reproduce

open the following code in vscode

pair :: IndexedFold Bool (a, a) a
pair p (a, b) = _ -- (,) <$> indexed p False a <*> indexed p True b

hover over the hole, then inspect other variables (e.g. a or b).

Expected behaviour

don't crash

Actual behaviour

HLS may be slow with some holes (lens is big) but it shouldn't crash. HLS tries to load but not showing the result (cpu and memory usage are super high), after a few tries it crashes.

[pepeiborra]

I suspect that it is GHC that is being slow and using lots of memory here. If that's the case, GHC 9.2 and newer will perform much better. Could you try and confirm?

[farmerzhang1]

GHC 9.2.1 does perform much better. But I see other strange things, for example, the type of term a should be a but it shows a really weird type a_a1vyB instead

[michaelpj]

(Related to the type variable issue: #2716)

[michaelpj]

It might also be good to revive #1914, hole-fitting will likely always be quite expensive.